We plan to develop a quantitative method for measurement of immune complexes by a solid phase radioimmunoassay in which I125 labelled rheumatoid factor will be used as the test reagent. With this technique we plan to examine sera and effusions from patients with rheumatoid arthritis and hypocomplementemia with respect to their content of immune complex like material and its relationship to the complement fixing property of these sera. We plan to partially isolate the material to immunize animals in the hope of identifying specific rheumatoid antigens and to work out the pathways by which the material activates complement. BIBLIOGRAPHIC REFERENCES: McDuffie, F.C., Hunder, G.G., Clark, R.J.: Complement Fixing Material in Sera of Patients with Rheumatoid Arthritis, Non-Identity with Rheumatoid Factors. Clin. Res. 22:643A, 1974; Luthra, H.S., McDuffie, F.C., Hunder, G.G., Samayoa, E.A.: Immune Complexes in Sera and Synovial Fluids of Patients with Rheumatoid Arthritis: Radioimmunoassay with Monoclonal Factor. J. Clin. Invest. To be published August, 1975.